1. Technical Field
The present invention relates to a material testing machine, and more particularly, to an electrohydraulic servo-controlled material testing machine.
2. Related Art
An electrohydraulic servo-controlled material testing machine is known which is comprised of a controller for supplying, via a servo amplifier, an electric signal input to an electrohydraulic servo valve provided between a hydraulic power source and a hydraulic actuator such as a hydraulic cylinder, the electric signal input varying in accordance with a target displacement of a movable part of the actuator. Typically, the quantity of fluid output from the servo valve varies in response to the electric signal input, and the movable part of the hydraulic cylinder is displaced at a speed proportional to the fluid quantity, whereby a load is applied to a test piece held between the cylinder movable part and a main body of the testing machine. An actual displacement of the cylinder movable part is detected and is supplied as a feedback signal to the controller. Under the control of the controller, a feedback control is carried out to cause the actual displacement to be close to a target displacement.
In this specification, the term "displacement of a test piece" indicates the displacement of one end of the test piece coupled to a movable part of an actuator which is caused by the displacement of the movable part of the actuator. In a material testing machine of a type provided with two actuators having movable parts which hold a test piece therebetween and are typically displaced in opposite directions, the term "displacement of the test piece" indicates the sum of displacements of opposite ends of the test piece caused by the displacement of the two movable parts of the actuators. That is, the term "displacement of the test piece" indicates deformation of the test piece caused by the displacement of the movable part(s) of the actuator.
The term "servo system" indicates a system mainly comprised of an actuator, a servo amplifier, and a servo valve. The term "control system" indicates a system mainly comprised of a servo system and a controller for controlling the operation of the servo system. The term "control loop" or "feedback control loop" indicates a loop mainly comprised of a servo system, a controller, and a test piece. Moreover, the term "force control system" indicates a control system for carrying out a feedback control with use of an actual force applied to the test piece, as the controlled variable, whereas the term "displacement control system" indicates a system for executing a feedback control using, as a controlled variable, an actual displacement of the test piece. The term "load" indicates a broadly defined load which includes a force applied to the test piece and generally referred to as a load, and which also includes the displacement of the test piece. If the actual load or the actual displacement is referred to as a controlled variable, the term "control objective value" or "control target value" indicates a target load.
In order to detect the controlled variable (the actual force applied to or the actual displacement of the test piece) in the feedback control performed in the testing machine, a detection amplifier which is mainly comprised of a highly sensitive analog amplifier is employed. The detection signal, indicative of the controlled variable and supplied from an analog detection system including such a detection amplifier, is liable to be affected by external disturbance such as electric power noise, especially when the controlled variable is small in magnitude, i.e., when the level of the detection signal is low. Thus, it is difficult to attain a sufficient signal-to-noise ratio, causing the control accuracy to be lowered.
In order to ensure an adequate signal-to-noise ratio (SN ratio), a multi-range detection amplifier may be employed which has plural ranges such as a .times.1 range, .times.2 range, .times.5 range, and .times.10 range and which permits range-selection to select a desired range, whereby the amplification factor (amplification gain) is changeable in dependence on the level of the detection signal. Further, as proposed in Japanese provisional patent publication no. 3-248033, the level of the control objective value (target load) is sometimes adjusted in synchronism with the range selection. Moreover, in order to eliminate turbulence in the control caused by a deviation between the timing of range selection and that of level adjustment for the control objective value, the arrangement disclosed in this patent publication is provided with a hold circuit which holds an error between the control objective value and the detected value obtained just before the range selection. However, the loop gain in the entirety of the feedback control system changes upon range selection for changing the amplification factor of the detection amplifier. To compensate for a change in the loop gain caused by the range selection, the feedback control system, including a controller configured by a digital circuit, requires reestablishment of the control gain when the range selection is made. Therefore, a large scale control must be made, if the control objective value is changed in synchronism with the range selection as proposed in the aforementioned patent publication. In addition, the range selection in the detection amplifier comprised of an analog amplifier generally requires much time in the order of several tens of milliseconds, whereas the level adjustment for the control objective value in the digital control system can be rapidly made within a time period in the order of 100 microseconds. Unstable operation of the digital control system can be found due to a large deviation between the completion timing at which the level adjustment in the digital control system is finished and the completion timing of the range selection in the analog detection system.
Prior to material testing, it is desired to stably mount a test piece onto the body of a testing machine. Especially, it is preferable to prevent the test piece from receiving an excessive load during the mounting work and from receiving an undesired load on and after completion of the mounting work. However, it is difficult for the operator to stably mount the test piece onto the machine body under his or her visual observations.
Heretofore, If that the test piece is mounted between stationary and movable chucks of the machine body, a first end portion of the test piece is mounted to the movable chuck coupled to the movable part of the hydraulic actuator of the machine, and then the hydraulic actuator is manually operated to cause a second end portion of the test piece to gradually move toward the stationary chuck in a condition that the actual force applied to the test piece is monitored by a load meter. When a slight actual force applied to the test piece is detected, it is determined that the test piece is brought in light contact with the stationary chuck, and then the second end portion of the test piece is grasped by the stationary chuck. However, the force generated when the test piece is brought in contact with the stationary chuck greatly varies depending on the stiffness of the test piece, and an excessive force may be applied to the test piece even if the displacement of the movable part of the hydraulic actuator is small in magnitude. In addition, the test piece mounted to the machine body is already applied with a force at that time. Thus, it is difficult to establish the initial state where the test piece is at no load.